Structurally related compounds are disclosed in U.S. Pat. Nos. 3,686,210; 3,801,593, 3,441,570; and 3,773,759, as well as the following references: J. Het. Chem., 6, 599 (1969); J. Med. Chem., 15, 762 (1972); Khim.Farm. Zh. 6, 11-13 (1972), CA, 78: 43182p; and Khim. Farm. Zh. 28, 23 (1968), CA 69: 106404Z.